One of the most fundamental principles of reasoning and
investigation is what has come to be known as Occam's Razor. Named
after the 14th century logician William of Occam, it is the principle
which favors the least complicated of two or more possible
explanations for an observation. Needless to say, most conspiracy
theories don't satisfy this rule.

In practice, Occam's Razor is used to cut away elements of
theories which cannot be observed. For example, Einstein described
space-time in the special theory of relativity. Lorentz had theorized
that space-time fluctuations are caused by motion through the "ether".
However, Lorentz's ether cannot be observed even though his equations
produce the same results as Einstein's, so it represents an
unnecessarily complicated model. It doesn't prove Einstein right and
Lorentz wrong, but because there's a whole lot less baggage to
Einstein's model, it's more likely to be correct given the current set
of observations.

Conspiracy theories generally entail the opposite of Occam's
Razor. That is, when explaining observations, the conspirators often
propose more complicated explanations than the commonly believed
story. Their conclusions often require us to believe in additional
postulated events or factors for which there is seldom any direct
proof. Occam's Razor clearly requires us to eliminate candidate
explanations which imply the existence of unobserved phenomenon.

Both NASA and the conspiracists offer explanations which fit the
observable phenomena. But some Apollo conspiracy theories require us
to believe in things like NASA death squads and top-secret soundstages
in remote locations. There is no direct evidence for either of those.
The possibility that these things -- if they existed -- might explain
the conspiracists' observations is not proof that those things
exist.

On a grander scale, conspiracists often have an elaborate
explanation for one photograph or statement and another completely
different but equally elaborate explanation for the next photo and so
on. Soon these piecemeal propositions start contradicting each other.
And you get different explanations depending on which conspiracist you
ask.

It's not suspicious that different conspiracists have different
ideas. That's how investigation works. But it is a big deal when one
conspiracist's theory, taken as a whole, propounds into a looming mass
of unfounded speculation. Instead of the typical process of looking
at all the possibilities and deciding which of them best makes sense,
conspiracists generally follow a line of reasoning which first demands
that the conspiracy exist. They then follow whatever tortured path of
conjecture is necessary to arrive at that conclusion.

The resulting line of reasoning may appear airtight. The reader
can follow the argument from first principles to conclusion. But the
reader often fails to ask whether that line of reasoning really is the
only possible one, and whether the conspiracist's argument requires
the reader to believe in extraneous propositions for which there is no
evidence.